Preheat oven to 350°F and prepare a 9×5 inch loaf pan with parchment paper.

Mix cinnamon and sugar for the swirl in a small bowl and set aside.

Mash bananas in a large bowl and whisk in melted butter.

Stir in granulated sugar and brown sugar until smooth.

Add eggs one at a time followed by vanilla extract.

In a separate bowl whisk flour baking soda cream of tartar salt and cinnamon.

Fold dry ingredients into wet mixture until just combined.

Pour half the batter into the pan and sprinkle half the cinnamon sugar mixture evenly.

Add remaining batter and top with remaining cinnamon sugar.

Swirl gently with a knife and bake for 55 to 65 minutes until a toothpick comes out with moist crumbs.

Cool for 15 minutes in the pan before transferring to a wire rack to cool completely.